Thursday, July 05, 2007

Crossroads Update

There is no change from the information discussed 3 July. This posting is only a very short update.

Tropical convective forcing continues to slowly increase across the warm west central Pacific Ocean centered ~0/145E while the intense rainfall persists with the South Asian Monsoon system. The MJO signal is very weak. Surface westerly winds having anomalies ~5m/s have developed west of the date line on the equator and will provide a source of cyclonic relative vorticity for upcoming TNWP tropical cyclones during the next few weeks.

It is now a case where we have 2 regions of intense Eastern Hemisphere tropical forcing. The enhanced South Asian Monsoon system favors GSDM Stage 1 while the west central-TNWP favors GSDM Stage 2. The latter has been recently dominating and circulation anomalies across the PNA sector are retrograding. However, global AAM tendency is barely positive (through 2 July per R1 data) with a contribution from the global frictional torque which has become weakly negative. Global mountain torque is still positive but decreasing. Thus the GWO is likely looping around on the left-hand (“La-Nina”) side of the phase space plot.

We may see this “tug of war” between the 2 regions of tropical forcing and subsequent circulation interactions the remainder of this summer. Which area will dominate is unclear. At least for the next week or so the central USA cool/wet regime is quite probable. There is another region of strong tropical convection developing southwest of India, and this area may propagate northeast replenishing the intense and severe thunderstorm activity now pounding regions from Pakistan into Southeast Asia. Meanwhile some consolidation of tropical forcing may occur weeks 2-3 as convective clusters shift toward the Asian Mainland from the west central Pacific. This speculation suggests additional retrogression of the ~west coast ridge-central USA trough- Deep South ridge wave train may occur.

Finally, Linear Inverse Modeling (LIM) work, a statistical technique used to predict variations in the ENSO cycle, suggests an evolution toward a well defined La-Nina by the October-December 2007 (3-month mean) period. See

for details. The recent trade wind surge has contributed to a large negative projection onto the optimal initial structure for anomaly growth. It remains to be seen how well this forecast will perform.

Appendix

An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively. The WB (2007) paper on the GSDM has been published in the February issue of MWR. It is likely I will not be able to post another discussion until early the week of 16 July due to travel. However, please check. In general, due to covering shifts and travel, my postings on this Blog will be irregular through at least August.